1. Field of the Invention
The present invention relates to a method for inhibiting polymerization of a vinyl compound. More particularly, the present invention relates to a method for effectively inhibiting the polymerization of (meth)acrylic acid or esters of (meth)acrylic acid during these storage, transportation and production.
2. Description of the Prior Art
Vinyl compounds such as (meth)acrylic acid (i.e., acrylic acid or methacrylic acid), esters of (meth)acrylic acid, and acrylonitrile are prone to be naturally polymerized by light or heat due to the vinyl bond they have. Therefore, there have been proposed to use various polymerization inhibitors in order to inhibit polymerization of (meth)acrylic acid or esters of (meth)acrylic acid during their storage, transportation or production process.
As an example, there has been proposed use of methoquinone. Japanese Examined Patent Publication No. 414121 discloses a method for inhibiting polymerization of vinyl compounds using an N-oxyl compound such as bis-(2,2,6,6-tetramethyl-4-piperidino oxyl) sebacate.
However, the inventors of the present application have studied the stability of a vinyl compound during storage or transportation after addition of the above-mentioned N-oxyl compound, and found that the concentration of the N-oxyl compound gradually decreases in contact with the vinyl compound, and it is difficult to prevent polymerization of vinyl compounds for a prolonged time.
In the production process of vinyl compounds, particularly in the processes of recovery, purification and synthesis of vinyl compounds, polymerization of vinyl compounds proceeds. Therefore, there have been proposed to use various polymerization inhibitors in order to inhibit the polymerization of (meth)acrylic acid or esters of (meth)acrylic acid during these processes.
For example, Japanese Examined Patent Publication No. 45-1054 discloses a method for inhibiting polymerization of acrylic acid using an N-oxyl compound such as tertiary-butyl nitroxide and 4-hydroxy-2,2,6,6-tetramethyl-4-piperidino oxyl alone, gives better polymerization inhibiting effect than that of the conventionally known hydroquinone, pheno-thiazine, and cupric chloride etc.
Japanese Examined Patent Publication No. 54-3853 discloses a method for inhibiting the polymerization using 4-hydroxy-2,2,6,6-tetramethylpiperidino oxyl or 2,2,6,6-tetramethylpiperidino oxyl in the production process of methacrylic acid from methacrolein using a gas containing oxygen in an organic solvent.
Japanese Examined Patent Publication No. 58-46496 discloses a method for inhibiting polymerization of (meth)acrylic acid or esters of (meth)acrylic acid, using 3-oxo-2,2,5,5-tetramethyl pyrolidino oxyl or 4-acetoxy 2,2,6,6-tetramethylpiperidino oxyl.
Chinese Patent CN 1052847A discloses a method for inhibiting polymerization of acrylic acid and esters of acrylic acid, using 4-hydroxy-2,2,6,6-tetramethylpiperldino oxyl alone or used in combination with hydroquinone, which is shown to be higher than the effect of combined use of copper dibutyldithiocarboxylate and hydroquinone.
Japanese Unexamined Patent Publication No. 6-345681 discloses a method for inhibiting polymerization of acrylic acid and esters of acrylic acid, using N-oxyl compounds such as 2,2,6,6-tetramethylpiperldino oxyl, and 4,4xe2x80x2,4xe2x80x3-tris(2,2,6,6-tetramethylpiperldino oxyl) phosphate in combination with phenol compounds such as hydroquinone, and phenothiazine compound.
Japanese Unexamined Patent Publication No. 9-316026 discloses a method for inhibiting polymerization of acrylic acid and esters of acrylic acid during its production, particularly during distillation process, using an N-oxyl compound in combination with phosphorus compound.
In the use of N-oxyl compounds according to the conventional polymerization inhibiting technology in the production process, for example, to prevent polymerization during distillation of crude acrylic acid, an N-oxyl compound is dissolved in acrylic acid and the solution is transferred to a distilling column by a pump.
The inventors found that the transfer of the solution, in which the N-oxyl compound is dissolved in acrylic acid, to the distilling tower in the recovery, purification and synthesis process of vinyl compound as mentioned above does not sufficiently inhibit the polymerization of vinyl compound.
It is an object of the present invention to provide a method for inhibiting polymerization of a vinyl compound which has overcome the problems residing in the prior art.
It is another object of the present invention to provide a method for stabilizing and inhibiting polymerization of vinyl compounds such as (meth)acrylic acid and ester of (meth)acrylic acid which can reliably assure stabilization and inhibition of polymerization of such vinyl compounds during storage, transportation and production of them, in the presence of an N-oxyl compound and a specific amount of water.
According to an aspect of the present invention, a method for inhibiting polymerization of a vinyl compound uses an N-oxyl compound and a specific amount of water. The presence of the N-oxyl compound can inhibit polymerization of the vinyl compound more effectively.
According to another aspect of the present invention, an N-oxyl compound is dissolved in water to produce a solution, and the solution is added to a vinyl compound in a process of recovering, purifying, or synthesizing the vinyl compound. In this way, polymerization of the vinyl compound can be effectively inhibited.
The vinyl compound has a vinyl bond which is liable to cause polymerization. Also, the N-oxyl compound includes any water soluble N-oxyl compounds.
The present invention is directed to inhibition of polymerization of (meth)acrylic acid and esters of (meth)acrylic acid.
Acrylic esters include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate and the like. Also, methacrylic esters include methyl methacrylate, propyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate and the like.
According to the present invention, any water soluble N-oxyl compounds is useable. It is preferable to use 2,2,6,6-tetramethylpiperidino oxyls represented by the following general formula (1); 
wherein R1 represents CHOH, CHCH2OH, CHCH2CH2OH, CHOCH2OH, CHOCH2CH, OH, CHCOOH or Cxe2x95x90O, R2 represents H or CH2OH.
The above-mentioned 2,2,6,6-tetramethylpiperidino oxyls include 4-hydroxy-2,2,6,6-tetramethylpiperidino oxyl, 4-oxo-2,2,6,6-tetramethylpiperidino oxyl, 4-carboxy-2,2,6,6-tetramethylpiperidino oxyl, and the like. Among these, preferably used are 4-hydroxy-2,2,6,6-tetramethylpiperidino oxyl, 4-oxo-2,2,6,6-tetramethylpiperidino oxyl, and the like, and particularly preferably used is 4-hydroxy-2,2,6,6-tetramethylpiperidino oxyl. These N-oxyl compounds may be used singly or at least two of these compounds may be used in combination. In this case, the mixing ratio may be appropriately decided.
More specifically, an N-oxyl compound may be preferably added in an amount of 0.0005 parts by weight or more, more preferably 0.001 parts by weight or more, to 100 parts by weight of a vinyl compound in order to ensure sufficient stabilization effects. On the other hand, addition of an N-oxyl compound too much causes staining in a produced acrylic acid. Accordingly, the amount may be preferably to set an upper limit up to 0.1 parts by weight, more preferably up to 0.03 parts by weight.
The adding amount of water may be adjusted in accordance with the solubility of water in the vinyl compound. It may be, however, preferable to dissolve 0.01 parts by weight or more of water to 100 parts by weight of the vinyl compound, more preferably 0.02 parts by weight or more, more preferably 0.05 parts by weight or more. To assure the inhibiting effect, on the other hand, it may be preferable to set the upper limit amount of water at 20 parts by weight, preferably 5 parts by weight, more preferably 2 parts by weight. If the adding amount of water is less than 0.01 parts by weight, it is difficult to regulate the concentration of the N-oxyl compound to a specified value, resulting in decomposition of the N-oxyl compound and consequently insufficient inhibition effect. While too much water will lower the stabilization effect thereof, as water promotes the polymerization of the vinyl compound, therefore the amount of water added shall be up to 20 parts by weight.
The addition of N-oxyl compound and water to a vinyl compound is not limited to a fixed sequence, but may be changed desirably. However, it may be preferable -that an N-oxyl compound is firstly dissolved in water and then the resulting N-oxyl compound aqueous solution is added to a vinyl compound. Also, it may be possible to add an N-oxyl compound into a mixture of a vinyl compound and water. In this way, there coexist three components, i.e., N-oxyl compound, vinyl compound and water. It could be presumed that this coexistence of the three components exerts the polymerization inhibiting effect.
The present invention may be applicable to vinyl compounds which contain impurities as by-product in the production process, or impurities contained in the starting raw materials of a vinyl compound. For acrylic acid containing various impurities, e.g., organic acids such as acetic acid, aldehydes such as acrolein, for example, the inhibiting effect can be attainable.
Further, it may be possible to use conventional polymerization inhibitors such as phenothiazine, methoquinone, copper dialkyldithiocarbamate, manganese acetate, and p-phenylenediamine, in addition to the inventive polymerization inhibitor.
Owing to the coexistence of an N-oxyl compound and a specific amount of water in a vinyl compound, the decrease in the concentration of the N-oxyl compound can be suppressed for a prolonged time. The coexistence of an N-oxyl compound and a specific amount of water in a vinyl compound delays the time for initiating polymerization more than the conventional inhibitors, thus inhibiting polymerization of a vinyl compound in the storage or transportation more effectively.
Further, a method for inhibiting -the polymerization, according to the present inventive method, during production of a vinyl compound will be explained in detail.
A vinyl compound in a process of recovery, purification and synthesis is kept from polymerizing by dissolving an N-oxyl compound in water, and adding the solution into the vinyl compound.
Conventionally, acrylic acid is produced from a reaction gas obtained by two stage catalytic oxidation reaction of propylene by the processes of: (1) collecting an acrylic acid in the form of an aqueous solution by making a reaction mixture gas containing acrylic acid in contact with water; (2) distilling the acrylic acid aqueous solution in the presence of an azo-tropic solvent and recovering crude acrylic acid; and (3) purifying the crude acrylic acid. The purifying process (3) includes distillation of separating substances having low boiling points such as acetic acid, distillation of separating substances having low boiling points such as unreacted acrolein, and distillation of separating substances having high boiling points.
The present invention is applicable for inhibition of polymerization of acrylic acid in any of the above-mentioned processes i.e., (1) collecting process, (2) recovering process, and (3) purifying process.
The addition of an aqueous solution of an N-oxyl compound is not limited to a particular manner, but may be carried out in a desired manner. For example, the aqueous solution of N-oxyl compound may be directly introduced into a vinyl compound in each process. In the case of purifying processor also, it may be introduced into in a supply line or a refluxing line.
An aqueous solution of an N-oxyl compound may be added only in the collecting process (1), or in each of the processes of collecting process (1), recovering process (2) and purifying process (3) (including various distillations). It may be preferable to add the aqueous solution in each process or each distillation stage.
In the case of adding -the aqueous solution in each process, it may be preferable to set the lower limit of adding amount of N-oxyl compound at 0.0005 parts by weight, more preferably 0.001 parts by weight, more preferably 0.002 parts by weight to 100 parts by weight of a vinyl compound in each process. The upper limit may be preferable to set at 0.1 parts by weight, more preferably 0.03 parts by weight, more preferably 0.02 parts by weight to 100 parts by weight of vinyl compound in each process.
If the adding amount of N-oxyl compound is less than 0.0005 parts by weight, the necessary polymerization inhibiting effect cannot be attained. In particular, if the operation temperature is more than 100xc2x0 C., the inhibiting effect noticeably lowers. On the other hand, if an N-oxyl compound is added more than 0.1 parts by weight, the product of acrylic acid is liable to be stained.
The amount of water in which an N-oxyl compound is dissolved may be changed in accordance with the kind and the amount of an N-oxyl compound used as well as the kind of process. However, it may be preferable to set the lower limit at 0.0005 parts by weight, more preferably 0.001 parts by weight, more preferably 0.002 parts by weight to 100 parts by weight of a vinyl compound in each process. If the amount of water is more than 10 parts by weight, on the other hand, the purity of the product of acrylic acid undesirably lowers. Accordingly, the upper limit for water may be set at 10 parts by weight, preferably 1 part by weight, more preferably 0.2 part by weight.
The concentration of an N-oxyl compound in an N-oxyl compound aqueous solution may be regulated in accordance with the kind of N-oxyl compound or the allowable amount of water. However, it may be preferable to set the upper limit at around 80 percent of the saturation solubility of the N-oxyl compound in water at a normal temperature to prevent precipitation of the N-oxyl compound in a tank storing the N-oxyl compound aqueous solution. On the other hand, the lower limit of the concentration of a N-oxyl compound depends on the amount of the N-oxyl compound to be added or the characteristic performance of an adding apparatus. However, it may be preferable to set the lower limit at around 0.1 weight percent.
The effect of inhibiting polymerization of a vinyl compound in the production process according to the present invention is not obtained simply because water coexists with the N-oxyl compound in each process or in each distillation stage. According to the present invention, regardless of the presence of water in the production process, the use of water as a solvent for N-oxyl compound, which means the N-oxyl compound is dissolved in water first and the aqueous solution is then added to the above-described process, is significant condition to exert the effect of inhibiting polymerization of a vinyl compound in the production process.
For example, in the collecting process (1), water is introduced to collect an acrylic acid. In the recovering process (2), the acrylic acid aqueous solution is introduced and distilled. Accordingly, water exists in each process location or tower. However, in the collecting process (1), the case in which an N-oxyl compound is added in the form of an aqueous solution inhibits polymerization of acrylic acid more effectively than the case in which the N-oxyl compound is added in the form of an acrylic acid solution (see Example 5 and Comparative Example 2). Accordingly, it could be seen that the addition of an N-oxyl compound in the form of an aqueous solution, which is in accordance with a method of the present invention, enhances the polymerization inhibiting effect of N-oxyl compound.
As mentioned above, in the collecting process (1) and the recovering process (2), there exists water. However, water is separated in each process. Accordingly, even if water exists in relatively large amount in these processes, there is no problem. On the contrary, in the distillation stage of -the purifying process (3), the amount of water should be kept in the specified range to prevent the purity of the product from lowering.
For example, in the collecting process (1) and the recovering process (2), the amount of water may be 1-1000 times by weight that of an N-oxyl compound, and in the purifying process (3), the amount of water may be 1-200 times by weight that of an N-oxyl compound in a distillation stage for separating substances having a low boiling point, and it is 1-50 times by weight in a distillation stage for separating substances having a high boiling point.
According to the present invention, molecular oxygen may be used concurrently, thereby enhancing the inhibition of polymerization of a vinyl compounds. As means of supplying molecular oxygen, it may be possible to adopt bubbling, or alternatively dissolve molecular oxygen in a solvent, and add the solution in a vinyl compound. For example, in the purifying process (3), molecular oxygen may be supplied from the bottom of a stripper tower or reboller in the form of gas. It may be preferable to supply molecular oxygen in an amount of 0.1 percent by volume or more with respect to the amount of evaporated vinyl compound. This is because of the fact that if it is less than 0.1 percent by volume, remarkable effect cannot be attainable. The upper limit of molecular oxygen to be supplied may be determined based the operation performance of distilling equipment. However, a large amount of supply requires reconstruction of the equipment. Accordingly, it may be preferable to supply molecular oxygen in not more than 1.0 percent by volume.
In addition to acrylic acid, the present invention may be applicable to other vinyl compounds.
The term xe2x80x9cproductionxe2x80x9d, xe2x80x9crecoveringxe2x80x9d, xe2x80x9cpurifyingxe2x80x9d, and xe2x80x9csynthesizingxe2x80x9d means the reaction process of a vinyl compound, and the above-mentioned processes (1)-(3). Further, such terms include a modification of the abovementioned process and any additional distilling process for separating a substance having a specified boiling point and the like.
In the case of acrylic acid, these terms respectively correspond to an oxidation process of propylene, recovering, purifying and synthesizing processes of acrylic acid. In the case of methacrylic acid, these terms respectively correspond to processes of separating, collecting, refining methacrylic acid from a reaction gas containing methacrylic acid obtained by catalytic gas phase oxidation of isobutylene. In the case of (meth)acrylic ester, these terms respectively correspond to processes of esterifying and purifying in addition to the above-mentioned processes of (meth)acrylic acid.